Typically, radar apparatuses are provided with an antenna that emits (transmits) a radio wave at a predetermined frequency in response to supply of electric power for emission, and receives the radio wave from the outside such as a reflection wave of the transmission wave. Typically, the antenna is installed outside. For this reason, it is necessary to provide a radome for covering the antenna to protect the antenna from external environment. The radome is a must especially for an antenna of a ship radar apparatus mounted on a ship because it is exposed to severe external environment.
JP2007-110201A discloses a structure of an antenna and a radome for covering the antenna. The radome of the antenna device disclosed in JP2007-110201A is formed in a substantially elongated cuboid shape. Inside the radome, an elongated waveguide antenna and a horn provided on the emission face side of the waveguide antenna are arranged.
However, in the conventional antenna device as disclosed in JP2007-110201A, in order to obtain desired vertical radiation pattern, a length in an emitting direction of a horn is needed to be about 3λ or more (here, a wavelength of the emission radio wave is λ). On the other hand, the horn also spreads in the vertical direction to some extent; however, the vertical direction does not require an opening length as much as an opening length in the emitting direction. Therefore, the horn has a long depth in a horizontal direction perpendicular to the elongated direction of the horn, and, on the other hand, it has a height in the vertical direction, which is not so long as the depth.
For this reason, the radome of the conventional antenna device is typically formed in an elongated shape, as well as a flat shape where the size of the radome is significantly large as compared with the size of a waveguide antenna, and the height is low and the depth is long. In addition, the weight of the antenna device including the radome is heavy.